The present invention generally relates generally to a grinder unit and a method for controlling it. The present invention more particularly relates to a grinder unit and a method for controlling a grinder unit that is provided with energy savings.
Effective solids conditioning is essential in order to achieve an efficient and reliable throughput in wastewater and sludge handlings systems. Therefore, various types of grinder units have been developed with the purpose of helping wastewater and sludge handling systems to operate more efficiently by reducing the particle size of solid waste material. The size reduction may be accomplished by a shearing, crushing, shredding action that is performed by one or more rotating elements of the grinder unit. Grinder units are typically used in wastewater and sludge handling systems. The grinder units play an important role because they reduce the particle size of solid waste material. In many applications this particle size reduction is required in order be able to use pumps for pumping wastewater and/or sludge.
Today, grinder units are normally built with counter rotating dual shaft design. A series of hardened cutters (like described in U.S. Pat. No. 5,511,729) are mounted on the dual rotating shafts. These cutters are configured to grind and hereby reduced the size of the solids.
Grinder units can be arranged in different configurations and normally they can be channel mount, wet-well, or as in-line pipe mounted. Traditionally an electrical panel is provided together with grinder unit. Inside the electrical panel there is the required emergency stop, breakers, fuses, etc. to form a safe electrical operation. Most electrical panels do also contain a Man-Machine interface with On/Off switch handle, audible and visual alarm indication in case of blockage, and display providing information on operational status of the grinder unit.
In order to reduce downtime of the grinder installation, some panels have a Programmable Logic Control (PLC) able to perform simple operation in case of blockage. Frequently, a current sensing device is continuously measuring the current of the grinder motor. When a blockage occurs, the current to the grinder motor will increase beyond a present level and the motor will be stopped. The PLC will then reverse the rotational direction of the grinder motor and the blocking device will be pushed back out from the cutting devices. When the reverse operation have been carried out for a short period the rotational sequence is shifted back to normal and grinding is continued. In case of running into overload situation (blockage) several times after each other—indicating that the grinder unit is not able to work its way through the solids—the operation will be stopped and normally an alarm will be given. But in many cases operational downtime is reduced as the grinder unit is able to overcome most solids, reversing a few times while cutting larger solids.
In most installation, the grinder units run continuously day and night, week after week, no matter whether solids are passing through the grinder or not. This leads to unnecessary high energy consumption.